First 3D sonar cloak could lead to invisible underwater mines

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With visual stealth technology finally seeming to make some progress, we might be tempted to ask: How can we be making progress in bending light, and still be so woefully inept at hiding from sound? Sound, after all, is a quantity we understand much better than light. It doesn’t flit about the quantum world with any of light’s annoying energy, nor does it travel literally as fast as anything could. Sound is simply ripples through a fluid, horribly slow by the standards of modern technology, and we still haven’t been able to figure out how to reliably avoid detection by sonar.

We do have a few countermeasures, like meticulously designed stealth vehicle bodies with angles designed never to reflect sound back to its source. In all, though, these solutions are imperfect, especially for submarines. A plane can generally expect detection waves to come from one general direction: below. A submarine, however, has to expect to be pinged from all angles, which makes simple wave deflection an inadequate solution. With radar, we can play with the wavelength, use coatings of precise thicknesses or with small disruptive balls, and cause light to scatter and interfere itself out of relevance. We have no such solutions for dealing with sound.

Now, a new study published in Physical Review Letters claims to have made a small plastic sphere invisible to sonar — but only sonar using a specific wavelength, and coming from a specific direction. The researchers are already mapping out future research allowing this concept to affect a range of wave sizes, and from a range of directions.

The technology works through setting up interference patterns, but where that can be achieved for radar using coatings and refractive materials, for sound we need something that can work in the medium of air pressure. Using a series of rings placed at mathematically generated intervals around the sphere, incoming waves can be dispersed and kept from bouncing back to their detector. A dizzyingly complicated series of reflections will occur, but the design makes sure every wave is cancelled out. As you might imagine, this solution would be somewhat awkward to integrate into aeronautical designs, but in the era of unmanned, supersonic drones, sonar has already lost much of its power over the air.

This is what Star Trek thought a cloaked mine would look like. Wrong again, Gene!

Underwater stealth has existed since the Second World War, first in terms of silencing mechanical noises from within a submarine, then with radar- and sonar-confusing coatings. None of it worked particularly well for very long, however, and today “stealth” submarines are more camouflaged than hidden. Submarines move through the very medium their enemies use to detect them; a series of rings that disrupts sound waves in water will also disrupt water flowing over the vessel, and change its hydrodynamic profile. We might envision a series of retractable rings that could be used when stationary.

Still, this demonstration sphere calls one application to mind: stealth mines. Roughly spherical and designed to be stationary in water for long periods of time, mines seem like the perfect application of this tech. Additionally, since mines are mostly used in bottlenecks and other areas that reduce the enemy’s movement options, the directionality of this tech might not be too debilitating; who cares if your minefield is visible from the port, if enemy subs will only ever be approaching from the ocean?

Not all applications need to be military in nature, of course. There’s no particular reason this needs to only dissipate sound waves coming from outside the cage; interference cages set up over noisy sites might someday filter out specific noises, like the whine of light rail lines or the rattle of early morning jackhammers. Most of our daily wave-based annoyances these days come from sound, rather than light, because we don’t live suspended in light.

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Elijah Thomas

It would be really cool if scientists could bend visible light, so it looks like its not there

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Magnus Blomberg

We do live in a medium carrying light the same as we do live in a medium carrying sound.

However the reason we are not more disturbed by light is tha facty that it is easily blocked while sound is not.

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